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on a barrel cactus flower |
nectar at a barrel cactus EFN |
On this page I will provide a brief species description and then discuss in turn:
Wheeler and Wheeler (1986) describe this species as being large to very large. Indeed, some major workers are over 1 cm in length putting them among the largest ants in the United States.
As indicated, C. ocreatus workers are polymorphic. Morphologically, major and minor workers differ in a number of ways. In general, mean body length of majors is 1.4 times that of minors (Lamon and Topoff 1981). A more striking difference, though, is the difference in the size and shape of the head: the heads of majors are twice as wide as those of minors and they are chordate in shape compared to the more rounded heads of minors. Finally, there are also color differences in the two castes (Wheeler and Wheeler 1986). The heads of major workers are reddish black while those of the minors are a lighter brown. The bodies of the majors are likewise generally darker in color.
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on a barrel cactus flower |
nectar at a barrel cactus EFN |
These ants are found in a number of different habitats. In addition to my desert site, Blom and Clark (1980) recorded them at their Rancho Santa Inés field site in Baja California. While Wheeler and Wheeler (1986) found them in both the Mojave Desert they also found them in the Pinyon-Juniper biome of Nevada. Interestingly, Andersen (1997) did not find any C. ocreatus workers at the two lowest elevation sites he sampled near Portal, Arizona. He did, however, find them at the next four higher sites that included desert scrub dominated by mesquite, oak-juniper woodland, and pine-oak woodland.
In the main, C. ocreatus is a nocturnal species. Workers at my field site left their nests soon after sunset and most returned well before sunrise. Occasionally, though, I did find a worker visiting a cactus during the day. But, marked workers who did this did it for only a few days before they invariably disappeared altogether.
Most of the workers leaving a nest visited EFN-bearing cacti, although at times they would also visit saguaros to collect pulp from recently opened fruits. Some majors, though, would hang around the nest entrance presumably to guard it (LaMon and Topoff 1981). Also, there were always a few workers who wandered around on the ground near the nest returning to it every now and then.
Workers visiting a particular plant seemed to travel independently of one another. They didn't travel as a group, took slightly different paths, and removing one ant didn't seem to affect the others. But occasionally I found pairs of ants engaging in something known as tandem running. In tandem running one ant usually leads another to a source of food or a new nest site. Tandem running has been reported before for C. ocreatus, but those who reported it did not elaborate on its function (Hölldobler and Wilson 1991, pg 273 table 7.7). I didn't study it in any detail either, but I can say the following of it. First, I saw minors leading both majors and minors. I never saw majors lead. Second, tandem running occurred very infrequently and I only saw any particular pair tandem run for a single night. Third, the leader of a tandem pair never led the follower directly to a food source or a new nest site. On one occasion a pair did eventually reach an EFN-bearing cactus but it was by a most circuitous route. Most of the time I gave up watching pairs before they reached any recognizable destination. Often such pairs would travel up and down small desert plants then head out in one direction only to return over the same ground a short time later.
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Given these observations, one possible explanation for tandem running in this species is that it plays some role in familiarizing new workers to their colony's landscape. Perhaps the "aimless" wandering of tandem pairs allows the trailing ant to familiarize itself with features of its landscape in relation to EFN-bearing cacti and the nest site. Among several possible tests of this hypothesis, an obvious one would be to determine if lead ants are older workers while the trailing ones are young and inexperienced. While I did mark some C. ocreatus workers, I didn't do it to the degree necessary to test this prediction.
While tandem running is not unique to these ants, they engage in another behavior that, at least to my knowledge, is. During the spring and summer I occasionally found clusters of workers on or at the base of cacti from which other workers were gathering food. Such clusters would form nightly for several weeks. Below are pictures of two clusters from different colonies.
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The majority of the workers in these groups were majors and, for the most part, they were fairly inactive The one thing the ants in these did do, however, was to take regurgitated food from both minors and other majors who had collected food farther up in the cactus. Below is a picture of a minor worker transferring its crop contents to a member of a cluster. This minor worker had just come down from the top of a saguaro where it had collected pulp from an open fruit. After regurgitating its crop contents to the major it then went back up the plant to continue feeding.
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Major workers frequently specialize in particular tasks. Often they are the primary colony defenders but, depending on the species of ant, may perform other roles as well. These roles include milling of seeds and storage of food (reviewed in Hölldobler and Wilson 1991). While both major and minor C. ocreatus workers will defend nests against army ant invasions, the majors with their large mandibles do most of the fighting (LaMon and Topoff 1981). Could it be that majors in this species also occasionally play another role -- that of mobile tankers?
There were at least 6 C. ocreatus colonies on my field site. Each had a single nest with a single entrance. The nest entrance of Colony 2 (see map below) was within 1 meter of a large Aphaenogaster cockerelli colony. In fact, it was located within the A. cockerelli's colony's "clearing", a space around the nest where extraneous debris is removed. Below is a map showing the 6 nests and the cacti visited by C. ocreatus workers during my study.
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Map of field site showing C. ocreatus nests and
cacti visited by workers of this species. The map shows
in red all the chainfruit chollas (*) and barrel cacti (0)
visited by C. ocreatus workers during the course of
my study. Included are not only frequently and heavily
visited plants, but also those visited only a few times by a
few workers. Another caveat is that visitation varied
greatly with season: during winter colonies were often
either inactive or only visited cacti near their nest
site. |
The above map reveals several things. First, workers can travel considerable distances to gather extrafloral nectar: those from Colony 3 traveled over 30 meters to collect extrafloral nectar from a large cholla. Even more striking is that colonies often pass up certain nearby cacti, in some cases very productive ones, to collect extrafloral nectar from more distant plants. I suspect that there are at least two reasons for this. First, while C. ocreatus workers can and do displace Crematogaster opuntiae workers (the most common EFN visiting ant at my site), they are most successful when there are relatively few C. opuntiae workers on a plant (see aggressive interactions between C. opuntiae and C. ocreatus). This may explain why C. ocreatus workers didn't visit chainfruit chollas at the eastern end of the field site: plants there were controlled by several large C. opuntiae colonies.
C. ocreatus workers may also simply prefer some EFN-bearing cacti over others. I measured circumference at 10 cm for 230 chainfruit chollas and found that plants visited by C. ocreatus workers were on average larger than those that weren't (Mann-Whitney U Test, p<.01). Of the six ant species investigated this was only true of C. ocreatus and the other Camponotus species, C. festinatus. If these ants are exhibiting a preference, it could stem from a difference in the amount of extrafloral nectar produced by different sized plants or from a difference in plant architecture between small and large plants (small plants are more spiny and this may inhibit movement by these relatively large ants). But a preference for large plants isn't the only potential explanation for why plants visited by C. ocreatus workers are larger than those that aren't. Another is that among cacti already occupied by other ant species, C. ocreatus workers may find it easier to access EFNs on larger plants (see aggressive interactions between C. ocreatus and S. aurea).
In the case of colonies 1 and 2, the above map also shows that workers from different colonies may simultaneously collect extrafloral nectar from the same cholla. In the one case where this happened workers from the two colonies generally behaved amicably toward one another. There were, however, occasional threat displays and even some instances of biting but nothing more severe than that.
Surprisingly, the most intense instance of aggression between workers of this species that I ever observed took place on the southernmost barrel cactus used by Colony 3, a plant almost certainly visited in the main by that colony only. That the one ant was temporarily attacked by a second suggests to me, though, that it was probably a rare visitor from another colony. At any rate, the whole affair still wasn't particularly violent. For the most part the two ants stood still and only occasionally tugged at each other or repositioned themselves. After about half an hour they separated and each went its own way.
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a potential intruder (right) |
temporarily joins in the attack |
C. ocreatus workers are fairly alert and avoid contact with humans. Likewise, I suspect that they retire from most large organisms and would be of little use in protecting EFN-bearing cacti from any vertebrate enemies these plants might have. I have, however, seen C. ocreatus workers catch moths attracted to my camera lights and both nymphs and adult coreid bugs avoid them (the nymph photographed at the end of a chainfruit cholla spine on the EFN web page moved out onto the spine after C. ocreatus workers came onto the plant).
Andersen, A.N. 1997. Functional groups and patterns of organization of North American ant communities: a comparison with Australia. Journal of Biogeography 24: 433-460.
Blom, P.E. and W.H. Clark. 1980. Observations of ants (Hymenoptera: Formicidae) visiting extrafloral nectaries of the barrel cactus, Ferocactus gracilis Gates (Cactaceae), in Baja California, Mexico. Southwestern Naturalist 25: 181-196.
Hölldobler, B. and E.O. Wilson. 1990. The Ants. The Belknap Press of Harvard University Press, Cambridge, Massachusetts.
Lamon, B. and H. Topoff. 1981. Avoiding predation by army ants: defensive behaviours of three ant species of the genus Camponotus. Animal Behaviour 29: 1070-1081.
Wheeler, G.C. and J. Wheeler. 1986. The ants of Nevada. Los Angeles County Museum of Natural History, Los Angeles. 138 pp.
Copyright (c) 1998 Barry Sullender
Rice University
All rights reserved